Japanese patent application publication No. 2006-129283A, hereinafter referred to as Patent literature 1, discloses a prior device management system for managing a plurality of devices via communication network. In the prior communication network, the communication network in the high hierarchy level is configured to manage the communication network in the low hierarchy level. Each the communication network in the low hierarchy level groups a plurality of the devices having functions same to each other. The communication networks are disposed in predetermined locations respectively. The communication network in the high hierarchy level is configured to select one of a plurality of the communication networks in the low hierarchy levels, and is configured to manage the communication network in the low hierarchy level. The prior communication network has a gateway. The gateway is configured to intermediate between the communication network in the high hierarchy level and the communication networks in the low hierarchy level. The gateway is provided with a protocol converter which is configured to convert a first communication protocol of the high communication network into a second communication protocol, and which is also configured to convert the second communication protocol into the first communication protocol. The first communication protocol is exemplified by BACnet (A Data Communication Protocol for Building Automation and Control Network). The second communication protocol is exemplified by LonWorks (registered trademark).
The gateway is provided with a plurality of communication units which is connected to the communication network in the low hierarchy level. The gateway is configured to activate one of the communication units selectively. This configuration makes it possible to manage a plurality of the devices over limitation of the number of the devices of the communication network in the high hierarchy network even if the number of the devices of the communication network in the low communication network is limited. In a case where the system is required to manage a plurality of the devices exceeding the limitation of the number of the devices of the communication network in the low hierarchy level, it is possible for the gateway to manage the devices over the limitation of the communication network in the low hierarchy level if the gateway has an empty communication unit.